This invention relates to the field of digital access arrangement circuitry. More particularly, this invention relates to digital access arrangement circuitry for connecting to a variety of phone line standards. The digital access arrangement circuitry may further include isolation barrier utilizing a capacitor coupled isolation barrier.
Direct Access Arrangement (DAA) circuitry may be used to terminate the telephone connections at a phone line user""s end to provide a communication path for signals to and from the phone lines. DAA circuitry includes the necessary circuitry to terminate the telephone connections at the user""s end and may include, for example, an isolation barrier, DC termination circuitry, AC termination circuitry, ring detection circuitry, and processing circuitry that provides a communication path for signals to and from the phone lines.
Generally, governmental regulations specify the telephone interface requirements and specifications for a variety of parameters including AC termination, DC termination, ringer impedance, ringer threshold, etc. For example, Federal Communications Commission (FCC) Part 68 governs the interface requirements for telephones in the United States. However, the interface requirements world wide are not standardized, and thus, in countries other than the United States the applicable standards may include the CTR21, TBR21, NET4, JATE, and various country specific PTT specifications. Because the interface requirements are not standardized from country to country, often different DAA circuitry is required for use in each country in order to comply with the appropriate standard. The requirement for different DAA circuitry, however, limits the use of one phone line interface in a variety of countries. Thus, for example, a modem in a laptop computer configured for interfacing with a phone line in one country may not necessarily operate properly in another country. Further, the requirement for different DAA circuitry in various countries hinders the design of a single integrated cost effective DAA solution for use world wide.
As mentioned above, the telephone interface requirements generally include specifications for the ringer impedance presented to the telephone line. Most countries and specifications merely have a minimum impedance which must be presented to the TIP and RING lines during a ringing event. However, some countries also have a maximum ringer impedance limit. In some cases these specifications may be contradictory. For example, before the implementation of the pan-European CTR21 specification, Germany required the ringer impedance presented at the tip and ring lines to be less than 20 kohms (at 25 Hz) while Austria required the ringer impedance to exceed 20 kohms (at 50 Hz). Even with the acceptance of the CTR21 in much of Europe, some countries still have maximum limits for the ringer impedance. For example, the maximum allowable ringer impedance (at country defined frequencies and rms line voltage) in the Czech Republic is 10 kohms, in South Africa is 60 kohms, in Poland is 30 kohms and in South Korea is 50 kohms. In the United States, the TIA/EIA 470B standard requires a maximum ringer impedance of 40 kohms at 20 Hz and 45 Vrms, however, this standard is generally not enforced.
One traditional approach to satisfy the various different ringer impedance requirements is to provide devices (such as discrete resistors and capacitors) between the TIP and RING lines external to the DAA integrated circuit. The values of discrete components may then be selected to meet the ringer impedance requirements for specific countries. Thus, the different circuitry may be required for different countries and circuitry meeting the ringer requirements of one country may not be suitable for use in other countries.
Thus, it is desirable to provide a DAA circuitry that may be suitable for use in many or all countries without the need for hardware or discrete device changes. Further, it is even more desirable to provide a DAA circuit that does not require the use of any external discrete impedance devices to provide the ringer impedance.
Further, it is also desirable that the DAA circuitry act as an isolation barrier since an electrical isolation barrier must exist in communication circuitry which connects directly to the standard two-wire public switched telephone network and that is powered through a standard residential wall outlet. For example, in order to achieve regulatory compliance in the United States with Federal Communications Commission Part 68, which governs electrical connections to the telephone network in order to prevent network harm, an isolation barrier capable of withstanding 1000 volts rms at 60 Hz with no more than 10 milliamps current flow, must exist between circuitry directly connected to the two wire telephone network and circuitry directly connected to the residential wall outlet.
Thus, there exists a need for reliable, accurate and inexpensive DAA circuitry for satisfying the ringer impedance standards for multiple country phone line standards and a DAA circuitry which also provides the necessary electrical isolation barrier.
The above-referenced problems are addressed by the present invention, which provides a reliable, inexpensive, DAA circuit that may be utilized with multiple telephone interface standards and which also provides an isolation system that is substantially immune to noise that affects the timing and/or amplitude of the signal that is transmitted across the isolating element, thus permitting an input signal to be accurately reproduced at the output of the isolation system.
The present invention provides digital direct access arrangement (DAA) circuitry that may be used to terminate the telephone connections at the user""s end to provide a communication path for signals to and from the phone lines. Briefly described, the invention provides a means for providing a proper ringer impedance for a variety of international phone standards. The invention may also be utilized with means for transmitting and receiving a signal across a capacitive isolation barrier. More particularly, a DAA circuitry may be utilized which satisfies many or all ringer impedance standards without the use of additional discrete impedance devices. The ringer impedance standards may be satisfied by use of an impedance structure coupled between the TIP and RING lines and actively controlling the current drawn through the hookswitch devices when a ringing event is detected so as to control the impedance seen at the TIP and RING lines during a ringing event. The detection of the ringing event may be performed on the phone line side of an isolation barrier. By controlling the current drawn from the phone lines with the hookswitch circuitry, an impedance placed in parallel with the TIP and RING lines may in effect be synthesized. In one embodiment the impedance of the impedance structure may be infinity (i.e. the impedance structure across the TIP and RING lines is not utilized). In this case, the current drawn through the hookswitch will sufficiently synthesize an impedance across (or in parallel with) the TIP and RING lines that dominates and effectively sets the impedance seen at the TIP and RING lines.
In one embodiment, a communication system is provided. The communication system may include phone line side circuitry that may be coupled to phone lines; powered side circuitry that may be coupled to the phone line side circuitry through an isolation barrier; and hookswitch circuitry within the phone line side circuitry. The hookswitch circuitry may draw current during ringer events to synthesize a ringer impedance.
In another embodiment, a method of operating a communication system is provided that may be coupled to a phone line. The method may include coupling an isolation barrier between powered circuitry and phone line side circuitry; providing hookswitch circuitry within the phone line side circuitry; and generating a substantial portion of a ringer impedance seen at the phone line by drawing current through the hookswitch circuitry.
A ringer impedance circuit within a communication system that may be connected to phone lines is provided in yet another embodiment. The ringer. impedance circuit may comprise a ringer detect signal; and at least impedance generating circuit coupled to the ringer detect signal, the at least one impedance generating circuit responsive to the ringer detect signal to provide a ringer impedance.
Yet another embodiment of the present invention includes a method of providing a ringer impedance capable of satisfying a plurality of ringer impedance standards. The method may comprise generating a ring detect signal; and adjusting the ringer impedance present at a phone line in response to the ring detect signal.
A method of generating a ringer impedance for a phone line is also provided. The method may comprise actively controlling at least one circuit of a DAA circuit; and substantially setting the ringer impedance present at the phone line by the active control of the at least one circuit.
In still another embodiment, an integrated circuit compatible with a plurality of phone line standards having ringer impedance requirements is provided. The integrated circuit may include a ringer detect signal; and at least one control signal generated in response to the ringer detect signal, the control signal operable to adjust the ringer impedance present at a phone line.